Disk drive data storage subsystems employ rotating disks and positionable data transducer heads in order to store vast quantities of data. The disks are conventionally mounted in a vertically stacked arrangement upon a common spindle hub. An internal DC brushless spindle motor rotates the spindle hub at a predetermined angular velocity. For high performance disk drives with high data transfer rates, it is common to encounter disk speeds in the 4000 to 6000 RPM category within 3.5 inch form factor disk drives.
A mass balanced rotary voice coil actuator motor is frequently employed to rapidly move the heads of a head stack in unison. The actuator moves the head stack from a departure track location to a destination location during track seeking operations. Once the head stack has arrived at a destination location, the selected head is settled over the desired data track, and by virtue of a track following servo loop "follows" the centerline of the track. The number of data tracks within a given area, known as track density or "tracks per inch" ("TPI") is directly a function of the accuracy of the track following servo loop, as well as the effective magnetic gap or track width provided by the head itself. Flying height also affects track width and TPI.
In order to achieve sufficiently narrow track widths, thin film heads are employed. When switching from write to read modes, these heads manifest for a brief interval a phenomenon known as Barkhausen noise. Barkhausen noise is believed to result from movement of magnetic domain boundaries in response to a changing magnetic excitation field. These movements are believed to be due to impurities and imperfections in the crystal structure forming the magnetic storage surface of the disk storage media. Barkhausen noise also appears to be one of the noise sources present in magneto-resistive ("MR") heads.
Disk drive architectures are known to employ embedded servo sectors. By "embedded servo sector" is meant one or more segments of a concentric data track which is recorded with head positioning overhead information, rather than with user data. In order for a head positioning servo loop to maintain desired track following, the loop must be provided with a sufficient number of position samples with each revolution. In addition, the servo loop must have sufficient servo information processing capacity in order to develop an accurate position error signal, and to apply that signal to the actuator motor to correct for off-track tendencies during operation.
Available data storage density per track increases as one moves from radially innermost data tracks to radially outermost data tracks. In other words, outer tracks are longer than inner tracks. Also, relative velocity between the rotating disk and the non-moving head increases from the innermost tracks to the outermost tracks across the disk surface. In order to optimize data storage, zoned data recording schemes have been proposed for many years. These schemes have involved varying the number of data blocks within radial bands or zones across the storage surface. An early teaching concerning zoned data recording is found in an article by Harold J. McLaughlin entitled "Disc File Memories" in Instruments and Control Systems, November 1961, 6 pages. A later example of a disk drive employing a zoned data recording arrangement is found in U.S. Pat. No. 4,016,603 to Ottesen. In the Ottesen approach, servo sectors were embedded at the beginning of each data block, and the data blocks of fixed length were not interrupted by servo sectors.
More recently, split data blocks have been proposed in which the embedded servo sectors interrupt and split up the data blocks into segments. One such proposal is found in Cirrus Logic Preliminary Data Sheet, CL-SH350, Integrated Synchronous SCSI Disk Controller, November, 1989. The Cirrus document described an integrated drive controller supporting split data fields under the direct, active supervision of a programmed microprocessor controller.
One drawback of disk drives employing thin film heads, embedded servo sectors and split data fields is directly related to the Barkhausen noise interval immediately following switchover from write to read modes. In other words, because of the Barkhausen noise interval, a track format is needed which minimizes overhead data and which minimizes transitions from write mode to read mode for minimal impact upon disk drive performance.